I've tried that. It works fine in koi ponds where there's a constant flow, but in AP solids settling filters it isn't effective because we only occasionally crack the valve to draw out waste. The solids simply settle, build up and then barely move when the drain is opened.
Better in that case to have the solids gathered into the center right on top of and in the drain pipe.

Yes. I didn't have room for a sump large enough to run these as a timed flood and drain. They are essentially wicking beds irrigated on the surface through a grid of 1/2" outlets. So far so good. The 4" of water/gravel underneath is enough to wick up to within an inch or two of the surface, close enough to wet the roots of young starts.
Beans started from seed directly in the sand had no problem germinating and sending roots down to the wet zone. Some shallow seeds however need to be watered by hand to get started.

I've been studying up a bit more on slow sand filters and discovered an interesting distinction between them and iAVS sand beds.....in slow sand filters the primary goal is to remove micro organisms, in iAVS it's to thoroughly digest wastes. As such, in slow sand filters the schmutzdecke is never allowed to dry out and when it does clog it is stirred up and the bulk of it drained off to waste somewhere else.
I guess I'm clarifying this in order to correct my inaccurate use of the term 'slow sand filter' when I actually meant nothing of the sort! =)
Somewhat unrelated, I'm currently experimenting with the iAVS method using small cinder in place of sand. I understand that the bacterial population won't be as high, but perhaps still sufficient. I'm curious to see exactly how it clogs. I know from experience that with larger cinder you still get a mat of biomatter on the surface, but that eventually the entire depth of the bed will clog due to the large void space. However, it is my hope that with small enough cinder (1/4") I can achieve similar results as sand, but at a lower cost. In particular, I'm thinking of eventually implementing this in Hawaii where there is no sand, but tons of cinder. Any sand you have seen there was either imported, unusable from coral, or simply broken down black cinder.
I'll share the results of this exploration as it progresses.

One not-insignificant drawback of the sand bed design is that large quantities of appropriately sized sand can be quite expensive. A recent build I completed (two beds: 3ft x 12ft, 4ft x 12ft) required 2 cu yds of 'playground' sand at $150 / cu yd! In the big picture $300 isn't the end of the world, but that particular sand was the most expensive aggregate sold at my local rock yard, 5-10 times more than gravel or lava.
The price is not a deal breaker per se, but for those looking to DIY something as cheap as possible, it is a factor.
Anyways, this has got me thinking.....
Sand is a great fine particle filter. I've been experimenting with a single 5 gal bucket full of sand to filter/digest the solids caught in the Radial Flow filter attached to my 500 gal koi pond. It was doing great while the temps were cool, but with the last couple warm months this little filter can't keep up with waste production. Appropriately sized though it would work just fine.
My thought is to use the iAVS sand bed concept as a fine filter and mineralization stage, but modularize it, separate from the growing area just like we've done with dedicated solids and bio filters in other systems. Perhaps an ibc or two of sand would be enough. Obviously it's more about surface area than depth but I'd need to find a compromise using conveniently available materials and techniques. Finding the minimum sand bed area needed for a given tank size and stocking density would allow me to use a cheaper medium like 1/4" cinder for the grow beds, or DWC or NFT.
This would give me many more growing options and setups, especially if I simply added the sand bed filter to the drain of the solids filter. My fish pump could keep going 24/7 keeping them happy and healthy.
Of course I'm aware I'm just reinventing the wheel here...implementing a slow sand filter and disassembling the iAVS concept to use only the features I need.

It is illegal and dangerous to shoot raccoons in city limits. Besides, who wants to stay awake all night standing guard?
Wicking beds don't attract them, but f&d gravel beds certainly do....they love the wet media. Having that pump on a timer set to 'off' at night should keep the surface dry enough not to attract them. We'll see.

I'm testing a half-measure version of the iAV sand bed on a client now...where only the small solids collected from the fines and bio filter will be sent to the sand bed. The solids caught in the RFF will be dumped into a soil garden, but always with the option to pour it into the sand beds.
The sand beds are 1ft deep, with the bottom 4" filled with gravel and acting as a wicking reservoir/sump.
Two serious issues I'm curious to see how they play out are:
1. cats using it as a litter box
2. raccoons digging up everything looking for worms